The present invention is directed to a conveyor load tracking system for tracking a load between processing stations, and more particularly to a system and method for tracking a load using the trailing edge of the load.
In the fields of material handling, industrial processing, and baggage systems, automated equipment is used to transport loads automatically through various processing steps. Typically, as loads move on a transportation system, such as conveyors, it is necessary to track each load and any data that is associated with a particular load as well as to control the conveyor. The conveyor may be made up of various conveyor segments, and it may be desirable to control each segment individually as well as track the load as it moves from conveyor segment to conveyor segment. For example, certain control requirements, such as destination station processing rates, load spacing on the conveyor, and selective destinations, may require the controller to start and stop conveyor segments independently, or independently vary conveyor segment speeds.
One common method used to track loads and control conveyors involves sensing the leading edge of the load. As the leading edge of the load is sensed, a load record associated with the load is created and selectively transferred by the controller into lists associated with each conveyor segment or area. Accurate tracking requires that the transfer of the records in the controller and data structure reflect the physical position of the load in the system. While this “leading edge” tracking technique is generally suitable for many applications, particularly when the loads have a common and consistent size and shape (e.g., pallet conveying systems), this technique is subject to ghost loads and race conditions as hereinafter described when used in systems transporting loads of varying sizes and shapes.
One problem associated with using the leading edge for both tracking and control purposes is that certain conveyor control conditions may lead to inaccurate tracking. For example, referring to FIG. 1(a), when a leading edge of a load X is sensed by a sensor B at the end of a conveyor segment C1, the system commonly transfers the load record to the list associated with the next conveyor C2. However, if control conditions require the stopping of conveyor C1 upon sensing the leading load edge of the load while continuing to drive conveyor C2, the encoder E for conveyor C2 continues to pulse and the expected window for load X moves forward on conveyor C2 even though the physical load is stationary at the discharge end of conveyor C1, thereby creating a ghost load in the list of conveyor C2.
One technique for addressing this type of ghost condition is to track both the leading edge and trailing edge of a load. When the leading edge of load X is detected by sensor B, the load record is updated and placed into a holding area associated with sensor B. Upon sensing the trailing edge of the load, the load record is released from the sensor B holding area to the list or data array for conveyor C2. This delay requires determining the size of the load in order to locate the leading edge of load X. To determine the size of the load, the number of encoder pulses is counted between the leading and trailing edges. One problem with this system is that it significantly increases the demands on the controller or processor. Therefore, to maintain operational efficiency, generally more complex and expensive controllers must be installed to track and control the system.
Leading and trailing edge approaches are also used with two sensor arrangements, such as that shown in FIG. 1(b). While this approach is appropriate in many instances, it breaks down if the maximum load length is greater than the spacing between sensors. That is, when sensor B2 detects the lead edge of the load, the controller will attempt to transfer the load record from the front of the list for conveyor C2 to the holding area for sensor B2. However, because sensor B1 has not yet detected the trailing edge of load X, the controller has not transferred the load record from the holding area for sensor B1 to the list for conveyor C2 and therefore no load record is present in the list for conveyor C2. In this condition, the system creates a new load record for the unexpected load. Therefore, when sensor B1 detects the trailing edge of load X, the load record is transferred from the holding area for sensor B1 to the list for conveyor C2. Since the expectation window for the leading edge of load X is calculated as being past sensor B2, the load record is removed from the model for failing to arrive at sensor B2. These additional steps of creating and deleting load records to compensate for loads having a maximum length greater than the spacing between the sensors further increases the demands on the controller or processor and increases the probability of errors in tracking.
The condition discussed with reference to FIG. 1(b) may be overcome by configuring the system such that sensor B1 does nothing when it detects the trailing edge of load X and, when sensor B2 detects the lead edge of load X, the system transfers the record at sensor B1 rather than the record at the front of the list for conveyor C2. However, as is shown in FIG. 1(c), this solution creates a race condition when the distance between leading edges of consecutive loads is smaller than the distance between sensors. For example, if two short length loads X and Y are traveling on the conveyor as shown, sensor B1 detects the lead edge of load X and updates the record to indicate that it is at sensor B1. As noted above, the controller does not update any records or lists when sensor B1 detects the trailing edge of load X. Therefore, if the leading edge of load Y is detected at sensor B1 before the leading edge of load X is detected at sensor B2, a collision occurs in the tracking model, specifically the load record for load Y attempts to overwrite the load record for Load X in the holding area for sensor B1.
In addition, as shown in FIG. 1(d), the above solution of ignoring the trailing edge at sensor B1 also breaks down if a strap, tag, or other loose item attached to load X is detected by sensor B1 but not by sensor B2 and the race condition noted above is satisfied. For example, sensor B1 detects the leading edge of load X and updates the record to indicate it is at sensor B1. Sensor B2 then detects the leading edge of load X and updates the record to indicate it is no longer at sensor B1 but rather at sensor B2. Next, the leading edge of a loose item attached to the trailing edge of load X is detected at sensor B1 and a new load record is created at sensor B1. Subsequently the loose item shifts and is not detected independently from the trailing edge of load X by sensor B2. As noted above, the controller does not update any records or lists when sensor B1 detects the trailing edge of new load, and since it is never detected at sensor B2, it remains in the holding area for sensor B1. When the leading edge of load Y is detected at sensor B1, a collision occurs in the tracking model as noted above
In summary, various methods have been employed with a limited degree of success to overcome and minimize the deficiencies in the prior art and a need exists for a simple solution that effectively tracks the load without adding additional control steps, while eliminating or reducing the potential for ghost loads, race conditions, or tracking collisions.